1. Field of the Invention
The present invention relates to a method for compression molding powder and a powder compression molded article made thereby. More particularly, it is concerned with a method for compression-molding powder to produce a molded article composed of a plurality of different material layers which are disposed in a compression direction, and a powder compression molded article produced thereby.
2. Description of the Prior Art
In powder metallurgy, mechanical parts having improved properties are produced at a low cost by using two kinds of powders.
Functional parts and structural members are generally produced by a method which comprises compression-molding powder into a predetermined form and firing or sintering the powder mold thus formed. This method is desirable and advantageous because the powder can be readily molded into any desired shape. Resin molded articles and sintered metal parts are produced by such powder compression molding methods. Generally, two layers consisting of different powder materials are arranged along the direction parallel to the pressurizing direction as described in Japanese patent publication no. sho-55-1961 and Japanese laid open application (OPI) No. sho-47-27814. Alternatively, two layers are arranged along the direction perpendicular to the pressurizing direction as disclosed in U.S. Pat. No. 2,753,858.
According to these methods, two kinds of powders are filled within the same die so that two powder layers can be subject to simultaneous pressure molding (compacting) to thus reduce production steps. However, in case complicated composite layers are to be molded as shown in U.S. Pat. No. 2,753,859, prior pressurization (compacting) of one of the powder layers is required.
In order to eliminate this additional process step, simultaneous pressure molding has been proposed wherein two kinds of powder materials are filled in the same die to integrally provide pressure molding and to provide a molded article having a complicated structure as disclosed in Japanese patent publication nos. 51-39166 and 54-31963. However, according to these methods, a plurality of lower punch means are required so that the punching means is weak in mechanical strength and complicated to operate. In addition, the mechanical wear of these punch means may degrade the dimensional accuracy of the resultant molded product.
As indicated, in powder compression molding, particularly in the production of functional parts, different materials are compression-molded in a multi-layer form to produce a molded article having special characteristics. This multi-layer construction is usually employed for the purpose of reducing material costs by using a special metal material or some other kind of special material for a predetermined layer or layers and an ordinary metal material or some other kind of ordinary material for the other layers. For example, in the case of a valve seat for use in an internal combustion engine, a composite sintered alloy is often used. The composite sintered alloy is composed of a high-alloyed sintered material and a low alloy sintered material. The high alloyed sintered material has good abrasion and corrosion resistance and is located on a valve spot surface of the valve seat and the low-alloyed sintered material forms the remaining portions of the valve seat. A composite material is also used when making resin parts for seals or bearings. The sliding surface of the resin part is made of a corrosion resistant or oil resistant material having a low coefficient of friction and the remainder of the resin portion is made of an ordinary material.
Such multi-layer powder compression molded articles have heretofore been produced most generally by a method and press machine shown in FIGS. 1(a) to (d). The press machine has a die 2, a lower punch 3, an upper punch 5 (FIG. 1(c)), a first feed shoe 6 (FIG. 1(a)) and a second feed shoe 7 (FIG. 1(b)). To produce an article according to the conventional method, a first powder A is introduced through the first feed shoe 6 by raising the die 2 relative to the lower punch 3 or lowering the lower punch 3 relative to the die 2. Then a second powder B is introduced through the second feed shoe 7 by again raising the die 2 relative to the lower punch 3 or lowering the lower punch 3 relative to the die 2. Thereafter, powder compression molding is effected with the upper punch 5 and the lower punch 3.
This method produces a valve seat as shown in FIG. 2(a ) and a resin seal ring 9 as shown in FIG. 3(a), each having a zone 81 or 91 made of a special material having specific desired characteristics.
The method shown in FIGS. 1(a)-1(d) inadequately reduces the volume of the special material required to produce the desired molded article and therefore does not adequately reduce the material costs.
In order to remove the foregoing defects in the abovementioned Japanese Patent Publication No. 39166/1976, a method is disclosed comprising the steps shown in FIGS. 4(a) to (d) using a press machine having a lower punch which comprises an inside lower punch 3A and an intermediate lower punch 3B. To make a molded article using this press machine, the inside lower punch 3A is first lowered to introduce a first powder A through a first feed show 6 and then the intermediate lower punch 3B is lowered to introduce a second powder B through a second feed shoe 7. Afterwards, an upper punch 5 is lowered to effect powder compression molding. This method permits one to obtain multi-layer powder compression molded articles as shown in FIG. 2(b) and FIG. 3(b) having only a part of the cross-section, i.e., the zone 81 or 91, made of the special material. However, as pointed out above, press machines having the above described double structure lower punches are mechanically weak in strength. Furthermore, when the inside lower punch and the intermediate lower punch do not fit in each other, satisfactory powder compression cannot be achieved and this impairs production stability. Also, since the lower punch comprises two punches 3A and 3B, the lower punch becomes complicated to operate and troubles often occur.